The present invention relates to a method and a device for removing coatings constituted by one or more layers deposited on the surface of a glass plate.
It is currently known to perform, during the manufacture of special glass plates, a treatment known as for "low emissivity", which substantially has the purpose of constituting a barrier to the passage of infrared rays, therefore limiting the outward transmission of heat from indoor spaces.
This treatment consists in applying a plurality of layers of material at the entire surface of a glass plate to be subsequently perimetrically associated, by means of a first seal of butyl sealant, at the lateral surfaces of a spacer frame, so as to form the inner chamber of an insulating glazing panel.
A typical sequence of layers is the following, starting from the surface of the glass plate, for the so-called "magnetron" quality: a first nonconducting layer of tin oxides that is strongly anchored to the glass and is approximately 300 angstrom thick.
A second conducting layer of silver, approximately 100 angstrom thick, and a third conducting layer of aluminum, approximately 35 angstrom thick, are applied over the first layer.
A fourth nonconducting layer of tin oxides is then provided, which is approximately 300 angstrom thick and has substantially protective characteristics.
Glass plates thus treated must be protected appropriately by means of a plurality of provisions, starting with packaging and ending with processing, until they constitute the double-glazing unit and therefore, since their treated surface faces the inside of the double-glazing unit, they are no longer exposed to the aggressive action of atmospheric agents.
However, the unalterability of said layers can be achieved only if the continuity of the coating is interrupted towards the outer rim of the glass plate; this continuity, even if atomic in thickness, would in fact be sufficient to trigger the oxidation of the outer rim of the glass plate towards the inside of the insulating glazing panel, consequently compromising characteristics that are indispensable for the good quality of the insulating glazing panel, such as the anchoring of the sealant that constitutes the second seal, the anchoring of the sealant that constitutes the first seal, the effectiveness of the "low emissivity" coating, and aesthetics.
Accordingly, the layers proximate to the perimetric rim of the glass plates are conventionally removed.
Accordingly, removal is conventionally performed by using heat as a layer-removing element; it is thus known to use a torch fueled with combustible gas, whose flame is passed at said rim of the glass plate.
However, this solution has now been abandoned, since an insufficient quality of the result has been observed in addition to objective difficulty in automating the process.
Removal is also conventionally performed by means of an electrical discharge (reference should be made to U.S. patent application Ser. No. 08/546,641); although this method is highly effective, also in view of the fact that it does not alter the surface of the glass plate in any way, it still has the limitation of a low feed motion speed.
In this field, it is also known to remove the layers at the rim of the glass plate by removing the layers by grinding, which is performed with a cylindrical grinding wheel the active part whereof is constituted by the outer generatrix, which is the only part that makes direct contact with the perimetric band of the glass plate.
The backing of the abrasive mix is usually constituted by rubber-like material having a low hardness value, so that the action against the surface of the glass plate is as tolerable as possible.
This conventional method substantially consists in passing over the perimeter of the glass plate with a cylindrical grinding wheel; the glass plate is placed on a horizontal surface so that the part to be treated faces upwards.
This conventional manual method has considerable drawbacks, since there is absolutely no control over the pressure of the grinding wheel against the surface of the glass plate except for the control based purely on the operator's sensitivity.
Furthermore, the resulting dust, which contains abrasive particles and other foreign particles constituted by the material of the mix, is a dangerous contaminant for the glass plate and particularly for its treated surface, which is highly sensitive, especially during the grinding operation known as edging, and in subsequent treatments, ranging from washing to the coupling of two or more glass plates to form the insulating glazing panel.
It is also known to perform removal by grinding with a semiautomatic method differing from the previous one in that the operation is still performed on a horizontal bed but the feed motion, too, is motorized in addition to the cutting motion.
Manual action, however, is still present in the operations for centering the glass plate and for starting and stopping the grinding machines on each side.
In both of these procedures, the effectiveness of dust aspiration is uncertain, due both to the low power levels installed on such machines and to the horizontal arrangement of the glass plate; accordingly, there are still considerable problems in terms of contamination of the glass plate surface.
Removal by automatic grinding is also conventionally performed, which occurs, again by virtue of the action of one or more cylindrical grinding wheels the active part whereof is constituted by the outer generatrix, on a glass plate arranged slightly at an angle with respect to the vertical plane; in addition to being performed fully automatically along the entire perimeter of the glass plate, either exclusively through the feed motion of the grinding machine or through the combination of the feed motions of the grinding machine and of the glass plate, it also introduces an attempt to control the pressure of the grinding machine against the glass plate surface.
However, this control is not easy to perform because of the size of the glass plate; because one must work at its rim and therefore in a boundary condition; because contact between the grinding wheel and the glass plate occurs only at the outer generatrix of the grinding wheel and therefore along a segment, not at a surface, also in view of the fact that any elastic deformability of the grinding wheel is not appreciable; because of the variability of the characteristics of the grinding wheel; because the diameter of the grinding wheel varies as a function of wear; and finally, because the glass may be abraded owing to the low thickness of the applied layers.
An example of the mentioned method is given by European patent EP 0165232, which, however, shows all the drawbacks mentioned above.
European patent EP 0 517 176 A1 is also known in which a method and a device for preparing a glass plate that is coupled to a multiple glazing unit are disclosed, wherein at least one plate is provided with a coating; from said patent a teaching is available that is based on the concept of using a grinding wheel that operates so that only its outer generatrix is in contact with the glass plate, and said method and said device are applied directly on a horizontal cutting bed, so as to remove the coating in the region that straddles the position of the cutting line and then cut the glass plate on the same machine.
All conventional removal methods are thus substantially penalized by the disadvantageous system for providing contact between the grinding wheel and the glass plate, with the consequent drawback of damaging the surface of the glass plate, which therefore has roughness and altered compositions which are not adapted for the correct anchoring of the first seal, which is constituted by a butyl sealant, and of the second seal, which is performed with polysulfide, polyurethane or silicone sealants; another drawback is that the active surface of the coating of the glass plate becomes contaminated with abrasive dust; yet another drawback is that it is difficult to adjust the force with which the grinding wheel presses against the glass plate.